<p>  Mold strength is an important control point for mold management. Shell mold strength varies under the influence of the resin coating that accumulates in sand grain contacts. Therefore, quantitative evaluation of the resin coating is important for mold management. In this study, in order to perform quantitative evaluation, we cut and ground shell molds to produce smooth coating surfaces. A scanning electron microscope was used to obtain backscattered electron images of the smooth surfaces. The cross-section of the resin coating and contact length of the sand coating containing sand grains (observed in the backscattered electron images) were measured. Due to the difference in the perimeter of each sand grain, the contact lengths of the resin coating with the sand grains varied. The results indicated a high correlation between “resin contact length/mean perimeter × 100” and mold strength. However, this correlation was negative for samples with varying blow pressure, and positive for constant blow pressure. This indicates that by varying the blow pressure and filling molds, different sized sites can be formed to make resin coating accumulate in the sand grain contacts. These sites were the first to affect the mold strength. Additionally, it was found that the resin quantity, resin type, silica sand type, silica sand grain size, artificial and special sand, and shell type at these sites are factors influencing the formation of resin coating and regulating mold strength.</p>